Position and orientation tracking systems (“trackers”) are well known in the art. For example, U.S. Pat. Nos. 4,287,809 and 4,394,831 to Egli et al.; U.S. Pat. No. 4,737,794 to Jones; U.S. Pat. No. 4,314,251 to Raab; and U.S. Pat. No. 5,453,686 to Anderson, are directed to AC electromagnetic trackers. U.S. Pat. No. 5,645,077 to Foxlin discloses an inertial system, and combination systems, consisting or two different trackers, such as optical and magnetic, are described in U.S. Pat. No. 5,831,260 to Hansen and U.S. Pat. No. 6,288,785 B1 to Frantz et al. Other pertinent references include U.S. Pat. No. 5,752,513 to Acker et al. and U.S. Pat. No. 5,640,170 to Anderson.
AC electromagnetic trackers have definite advantages over other types of systems. For one, AC trackers provide the highest solution/update rate with the greatest accuracy, not affected by obstructed field of view, in contrast to optical solutions. AC trackers do not require reference sensor/unit and drift stable apparatus of the type required by inertial units, and they are not affected by the Earth's magnetic field and the magnetization of ferrous materials, in contrast to DC magnetic systems.
The main disadvantage of AC trackers is that they are quite susceptible to distortion due to eddy currents in conductive materials in or near the motion box where precise tracking is desired. Eddy currents are a major cause of magnetic tracker inaccuracy. Methods of dealing with the problem are various. One method is to create a map of measured transmitter-receiver coordinates versus known coordinates. This method works very well in terms of accuracy achieved, but special fixturing is required and once performed the environment cannot be altered.
A pulsed DC tracker uses a method of sequencing DC current pulses to the transmitter then waiting during each pulse for eddy current effects to decay before recording field measurements. This method offers only slow update capability and suffers from earth's magnetic field interference and noise associated with low frequency powered appliances and equipment.
Yet another tracker method uses very low AC frequencies where eddy current interference is a small part of the tracker fields, but like the pulsed DC method, suffers from slow update capability and susceptibility to low frequency noise. Still another tracker method models eddy current conductors to compensate for measured fields enabling the use of tracker frequencies well above equipment and appliance noise band. This tracker method works well in the sense that it avoids the low frequency noise band, but it does not work well in the sense that it requires two tracker modes—a normal tracker operation mode and an environment testing mode complete with additional hardware. The testing mode suspends normal tracker operation while measurements of extended conductor characteristics are made using geophysics techniques.
A widely used technique for dealing with magnetic field distortion is to map the operating area of the tracker. In this approach, the system is set up with a fixed transmitter location that produces a known area of operation for the tracker. The operating area is then broken into a three-dimensional grid, and magnetic field data are gathered for each point in the grid. The gathered data provides a direct relationship between field data and position that is then used to compensate for the distorted environment using a variety of well known computational methods. This approach works well for moderately distorted environments but requires special equipment and extensive on-site preparation to generate the map. Changes of the operating area that significantly affects accuracy may require remapping.
U.S. Pat. No. 6,172,499 is directed to an AC magnetic position measurement system that compensates for eddy current distortion by taking advantage of the fact that the quadrature term of the sensor response is determined solely by the secondary field. The system measures the in-phase (I) and quadrature (Q) components for two different frequencies and uses them to select a compensation coefficient from a look up table. The compensation coefficient is used to scale the quadrature term (Q) before it is subtracted from the in-phase (I) term. The in-phase term (I) is then used for the position calculation. This algorithm is repeated for each axis of the system, requiring six separate frequencies for the highest resolution system. A second three frequency system is also proposed which reduces the system complexity but at the cost of reduced accuracy of the compensation coefficient. This method does not address the induced magnetization of ferrous materials and requires two separate frequencies for each axis unless reduced accuracy is acceptable.
U.S. Pat. No. 6,528,989 compensates for conductive distortion by computing a correction term based on the inductive limit and phase delay of the environment. The inductive limit and phase delay are calculated using either DC pulse measurements or multiple AC frequencies. Relevant theory can be found in Grant, F. S., and West, G. F., 1965 Interpretation Theory in Applied Geophysics (McGraw-Hill Book Company). The use of DC pulse measurements severely limits the throughput rate of the tracker due to the extended period of time required for eddy currents to decay (˜5 ms according to the author). The multiple frequency technique uses a wide range of frequencies in the magnetic field, requiring the use of a non-resonant transmitter which effectively limits the output power of the system which in turn limits useful range.
Despite these advances, the need remains for apparatus and methods of compensation for spurious, eddy-current-induced fields in AC electromagnetic tracking systems.